1. Field of the Invention
The invention generally relates to a flow conditioning device and more particularly is concerned, for example, with an axial-orifice type element that steps down the pressure within an axially flowing liquid regime. Specifically, the invention suppresses noise and instabilities caused by pressure fluctuations, vortex shedding, and cavitation that typically result when the pressure within an axial flow is reduced. The invention employs viscous dissipation to step down pressure within an axial flow, straightens an axial flow, and damps out free-stream instabilities within an axial flow. The invention produces a stable axial flow devoid of pressure and velocity fluctuations.
2. Background
A single-hole orifice 1, as shown in FIG. 1, is commonly used to meter flow, to damp acoustic resonances, or to reduce pressure over a small axial distance in a piping system. A single-hole orifice 1 typically includes a cylindrical outer wall 2. A plate 3 is attached to the interior surface of the outer wall 2 so as to completely traverse the flow path formed by the outer wall 2 thereby defining an upstream side 6 and a downstream side 7. The plate 3 includes a hole 4 that restricts flow of a fluid 5 from the upstream side 6 to the downstream side 7.
The performance of a single-hole orifice 1 is typically characterized by the average discharge coefficient representative of the ratio of the measured and theoretical volumetric or mass flow rates. The device is sized for use within a system based on the Reynolds Number of the flow, mass or volumetric flow rate, and the required pressure drop.
FIGS. 2a and 2b illustrate the pressure and velocity across a single-hole orifice 1, respectively. The pressure of the fluid 5 drops significantly from P1 to Pthroat and the velocity of the fluid 5 increases from V1 to Vthroat as a result of flow acceleration within the constriction of the orifice. The pressure recovers from Pthroat to P2 after the fluid 5 exits the constriction; however, P2 is lower than P1 because of losses due to flow resistance, frictional losses, and flow turning.
As seen in FIG. 1, hydrodynamic instabilities are formed when the fluid 5 traverses the hole 4 creating a vortex 8 attached to the plate 3 at the downstream side 7. The vortex 8 periodically detaches from the plate 3 in a process referred to as vortex shedding. The plate 3 introduces large shearing stresses into the fluid 5 during flow acceleration as the fluid 5 negotiates the hole 4. Shear is responsible at least in part for the periodic shedding of vortices. The flow acceleration results in a lower pressure within the fluid 5 through the plate 3. In some cases, the pressure can fall below the vapor pressure resulting in a vapor cavity or cavitation 9, as represented in FIG. 2a with the corresponding velocity increase in FIG. 2b. In other cases, the pressure in the vortex cores falls below the vapor pressure resulting in the generation of bubbles.
Pressure fluctuations from both the cavitation and the vortex shedding are amplified if the frequency is sufficiently similar to the natural frequency within the piping system. Sometimes it is possible for a single-hole orifice 1 to operate in a “choked flow” condition during high mass flow rates so that the pressure drop causes large vapor cavities to form. Flow transients in a piping system can disrupt the vapor cavities causing shedding and convection of vapor clouds in the downstream section of the piping system. The local temperature gradients and pressure recovery downstream can cause the clouds to condense producing violent high amplitude pressure spikes that damage the piping system. For some single-hole orifices 1, an unstable feedback loop can form between vortex shedding and acoustic perturbations originating from upstream components resulting in an amplification of the modes that convect downstream.
A single-hole orifice 1 is usually one component within a multiple component piping system that could include, but is not limited to, valves, pumps, turning ducts, and diffusers. As such, a single-hole orifice 1 rarely operates under nominally “steady” conditions and therefore is subject to pressure and/or velocity fluctuations. Even during non-cavitation conditions, small perturbations in the upstream side 6 elicit a highly complex non-linear dynamic response from a single-hole orifice 1 resulting in large scale fluctuations that are convected downstream.
At least one source postulates that a mode conversion takes place in acoustically-modulated, confined jets through an orifice resulting in a feedback instability. Another source reports that the overall response of an orifice is bounded by the response predicted by the one-dimensional linearized theory, the exception being a local resonance condition when the driving frequency is close to the natural frequency of the Kelvin Helmholtz instability in the orifice. Regardless, instability modes, either initiated or amplified, can have a profound effect on the operation of components downstream from a single-hole orifice. Various attempts have been made to inhibit and to control the hydrodynamic instabilities associated with a single-hole orifice.
In one example, a globe-style control valve with anti-cavitation trims was substituted for an orifice within a piping system including an orifice and a control valve. The globe-style control valve provided the functionality of an orifice and operated in choked flow much like an orifice. This approach reduced the extent of cavitation within the piping system; however, the globe-style control valve did little to reduce vibrations within the system. Furthermore, cavitation in the globe-style control valve eroded the valve trims increasing the risk of significant damage to the piping system over time.
In another example, a two-stage orifice was implemented to operate in conjunction with throttle valves within an emergency cooling and containment system applicable to the downstream section of a pressurized water reactor. A first stage caused fluid to flow through tangential slots while a second stage caused fluid to flow both axially and tangentially. The two-stage orifice achieved the resistance of a single-stage orifice without the cavitation of the latter. However, the tangential flow inherent to the two-stage orifice caused significant swirl in the downstream fluid that accelerated erosion of the piping system. Furthermore, the additional stage increased the overall length of the orifice such that the device was incompatible with many applications.
As is readily apparent from the discussions above, the related arts do not provide a device that minimizes the instability modes associated with pressure step-down functionality. In particular, the related arts do not describe a device that avoids vortex shedding and cavitation. As such, the related arts are prone to vibrational responses that compromise the structural integrity of a piping system and to flow conditions that erode downstream components within a piping system.
Accordingly, what is required is a flow conditioning device that achieves the resistance required for a particular pressure drop within an axially efficient design envelope.
Accordingly, what is also required is a flow conditioning device that suppresses the instabilities, namely, vortex shedding and cavitation, associated with the reduction of pressure within an axial flow.
Accordingly, what is also required is a flow conditioning device that minimizes pressure fluctuations in a downstream flow.
Accordingly, what is also required is a flow conditioning device that minimizes vibrations communicable to a piping system.
Accordingly, what is also required is a flow conditioning device that provides flow resistance or pressure drop while minimizing the risk of cavitation in cryogenic and volatile liquids with vapor pressures higher than conventional liquids, one non-limiting example of the latter being water.